Image projectors are common. These projectors are aimed at a vertical display surface to show a “slide” presentation or a video. Many of these projectors use transmission LCDs, and typically have a single lens. The projectors can display images one at the time or as a sequence of images.
These projectors are typically designed so that undistorted images are displayed on the display surface only when an optical axis of the projector is aligned perpendicularly to a center of the display surface. If the above assumption is violated, then the resulting output image may not be rectangular, and will be, at best, a trapezoid, and at worst an arbitrary quadrilateral. This problem is called keystoning.
With prior art projectors, the one way to correct keystoning is to tediously adjust the physical position of the projector by moving it around by translating and rotating the projector until a rectangular image is displayed.
U.S. Pat. No. 5,548,357, “Keystoning and focus correction for an overhead projector,” issued to Appel et al. on Aug. 18, 1998, describes a system where a test slide is displayed. A user identifies line pairs most parallel to each other. The line pair identification activates a distortion correction program that uses the oblique angle between the horizontal plane through the projector and the viewing screen.
U.S. Pat. No. 5,795,046, “Method for pre-compensating an asymmetrical picture in a projection system for displaying a picture,” issued to Woo on August 1998, describes a system where the projection angle, and the trapezoidal error, is compensated for by the user entering in positional information into the system via a keyboard.
U.S. Pat. No. 6,520,647, “Automatic keystone correction for projectors with arbitrary orientation,” issued to Raskar on Feb. 18, 2003, describes a method that corrects keystoning in a projector arbitrarily oriented with respect to a display surface. An elevation angle, a roll angle, and an azimuth angle of an optical axis of the projector are measured with respect to the display surface. A planar projective transformation matrix is determined from the elevation, roll, and azimuth angles. A source image to be projected by the projector is warped according to the planar projective transformation, and then projected onto the display surface.
However, digitally warping an image results in a degradation of image quality, because pixels have to be re-sampled, and some pixels are discarded.